Hot runner systems are utilized in injection moulds to introduce a plastic compound capable of flow at a pre-specified temperature into a divisible mould body (cavity) under high pressure. A heating system is usually provided to prevent the hot compound from cooling prematurely in the distribution channels and nozzles by maintaining the fluid plastic at a constant temperature. Temperature control requirements in hot runner moulds are extremely high, as a large percentage of the plastics being processed only have a very narrow processing range and react with extreme sensitivity to temperature fluctuations, particularly in the nozzle and gate area. This means, for example, that a temperature change of only a few degrees in the nozzle area suffices to cause injection errors and leads to rejects. Precise temperature control is therefore important if a fully-automatic hot runner mould is to function correctly.
Monitoring and controlling of temperatures is normally realized by temperature sensors in the form of electrical resistive conductors (resistance sensors). These are fitted as separate elements in grooves or holes integrated in the material tube or a heating block, e.g. as disclosed in EP-A1-0 927 617 or DE-U-201 00 840. Moreover, DE-A1-199 41 038 and DE-A1-100 04 072 also indicate that both the temperature sensor and the entire heating device can be realized through use of thick-film technology, i.e., by direct coating through application on the surface of the nozzle or manifold body.
The resistance sensor generally possesses a resistance element of U-shape or meandering form and made of metal or metal alloy which alters its electric resistance if temperatures rise or fall. However, the measuring technique associated therewith has the disadvantage of being only capable of recording a mean temperature value within a relatively broad spatial range, despite the sensor being carefully positioned in the hot runner system. It is therefore difficult to achieve the necessary degree of heat control for an exact temperature distribution at or near the end of a manifold, at a nozzle tip, and the like. Particularly injection moulds, however, require exact knowledge of the temperature at the nozzle tip so that the temperature can be exactly maintained and corrected if necessary.